1. Field of the Invention
The present invention relates to a method for estimating the alignment mark deformation of a processed wafer, a method to estimate a relationship between alignment mark deformation and overlay errors, a method for predicting a position of a desired point on a processed wafer, an alignment system capable of performing one or more of the methods, and a lithographic apparatus comprising such a alignment system.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
With the aid of a lithographic apparatus, different mask patterns are successively imaged at a precisely aligned position onto the substrate. The substrate may undergo physical and chemical changes between the successive images that have been aligned with each other. The substrate is removed from the apparatus after it has been exposed with the image of at least one mask pattern, and, after it has undergone the desired process steps, the substrate is placed back in order to expose it with an image of a further mask pattern, and so forth, while it must be ensured that the images of the further mask pattern and the subsequent mask patterns are positioned accurately with respect to the at least one already exposed image on the substrate. To this end, the substrate is provided with alignment marks to provide a reference location on the substrate, and the lithographic apparatus is provided with an alignment system to measure the alignment position of the alignment marks. By measuring the alignment position of the alignment marks, in principle the position of every point on the substrate can be predicted, i.e., the location of a previously exposed target portion can be calculated and the lithographic apparatus can be controlled to expose a successive target portion on top of the previously exposed target portion.
Usually, the alignment marks on the substrate are diffraction gratings. The alignment system then comprises an alignment sensor system with a light source to emit light towards the grating and a detector to detect the diffraction pattern in the reflected light, i.e., sub-beams diffracted in a first, third and/or higher order are used, in order to determine the position of the grating.
The processing of the wafer may render the alignment mark asymmetrical, so that the measured point of gravity of the grating no longer coincides with the geometrical center of the grating. This is called alignment mark deformation. Hence, when the alignment position after processing is measured, the alignment mark deformation introduces measurement errors when predicting the displacement in a desired point of the substrate.
If the predicted displacement in a desired point of the substrate is used to expose the substrate with a next pattern, the measurement errors result in a poor overlay performance.